Engine compression ratio in an internal combustion engine has long been recognized as an important control parameter for obtaining optimum and consistent engine performance. Knowledge and control of compression ratio is important, for example, in controlling engine "knock", in controlling exhaust emissions and in achieving desired mileage performance. Hence, the ability accurately to determine compression ratio during the manufacturing process is of increasing interest and importance.
Engine compression ratio may be expressed by the fraction (V.sub.DP +V.sub.TDC)/V.sub.TDC, where V.sub.DP is total displacement volume of the piston within the cylinder and V.sub.TDC is cylinder clearance volume or head space at piston top dead center (TDC). Even if the intake valve closes after piston bottom dead center, the above definition is often used for analytical purposes. Piston displacement volume or V.sub.DP may be accurately obtained or determined from the mechanical design parameters and tolerances of engine component manufacture. However, clearance volume at TDC cannot be so obtained with desired accuracy, and the art has yet to propose a method for accurately measuring V.sub.TDC in an assembled engine with the necessary and desired accuracy.